1. Technical Field
The present invention relates to a capacitance type load sensor for detecting load based on a change in capacitance caused by the load applied thereto.
2. Related Art
Conventional capacitance type load sensors comprise a pair of electrodes that are spaced and electrically insulated from each other to constitute a capacitor, and detect load on the basis of that change in the capacitance of the capacitor which is caused when one electrode applied with the load is flexed to change the distance between the electrodes.
FIG. 11 shows, by way of example, a conventional capacitance type load sensor which comprises an insulating board 1 formed with an electrode pattern 2 and an electrode plate 3 made of stainless steel, for instance. The insulating board 1 and the electrode plate 3 are spaced from each other, with spacers 4, 5 interposed therebetween, and are connected with each other by means of rivets 6 and 7, to thereby constitute a capacitor. Lead wires 8, 9 are connected to the electrode pattern 2 and the electrode plate 3, respectively. When load is applied to the electrode plate 3 in the direction shown by arrow P to cause the electrode plate to be flexed so that the gap G between the electrode pattern 2 and the electrode plate 3 changes in magnitude and hence the capacitance of the capacitor changes, the load sensor detects such change in capacitance, as the load applied to the electrode plate through the medium of the lead wires 8, 9.
In the load sensor having the aforementioned construction, the measurement accuracy becomes higher with the increase in an amount of change in capacitance, i.e., an amount of flexure of the electrode plate 3 caused when load is applied. On the other hand, stress generated in the electrode plate 3 becomes greater with the increase in amount of flexure of the electrode plate 3. If the generated stress exceeds the elastic limit, fatigue limit or impact breakdown limit, the durability of the electrode plate 3 is lowered, so that the durability and reliability of the load sensor, especially long-term reliability thereof, may be worsened. As apparent from the foregoing explanation, when load is applied, the electrode plate 3 is desirable to be flexed by a large amount from the viewpoint of improving the measurement accuracy, but to be flexed by a small amount from the viewpoint of improving the reliability.
That is, in a load sensor for detecting load based on a change in capacitance of a capacitor comprised of an electrode pattern 2 formed on an insulating board 1 and an electrode plate 3 that is adapted to be flexed when the load is applied, a conflict arises between a solution for improvement of measurement accuracy and that for improvement of reliability, and hence it is difficult to improve both the measurement accuracy and the reliability at the same time.
An object of the present invention is to provide a capacitance type load sensor which is high in measurement accuracy and long-term reliability.
A capacitance type load sensor of this invention comprises a first electrode, a second electrode disposed to face the first electrode and to be spaced therefrom and cooperating with the first electrode to form a capacitor, first and second holding members made of an insulating material, and an elastic member having opposite end portions thereof associated with the first and second holding members, respectively. The first and second holding members support the first and second electrodes for relative motion in the direction along which a facing area between the electrodes changes, with the distance between the electrodes kept constant. The elastic member is elastically deformable in the direction of the relative motion between the first and second electrodes. Load to be measured by the load sensor is applied to the first and second holding members or to the elastic member.
In the load sensor of this invention, load is applied to the first and second holding members or to the elastic member, and hence no load is applied to the first and second electrodes. Thus, there is a low possibility of causing deformation or damage of the electrodes upon application of load, and accordingly the load sensor is excellent in long-term reliability. When the elastic member is elastically deformed upon application of load, a relative motion occurs between the first and second electrodes, respectively supported by the first and second holding members with which the opposite end portions of the elastic member are associated, with the electrode distance kept constant. Thus, a change occurs in the facing area between the electrodes and therefore the capacitance therebetween changes. The amount of change in the capacitance accurately indicates the magnitude of the load applied to the load sensor, so that the measurement accuracy of the load sensor is excellent. In this manner, the present invention provides a capacitance type load sensor which is high in measurement accuracy and long-term reliability.
In the present invention, preferably, the first and second electrodes are each formed into a hollow cylinder shape which is open at its at least one end face. The second electrode is disposed coaxially with the first electrode. More preferably, the first and second electrodes are each formed into a hollow circular cylinder shape.
With this preferred embodiment, the facing area between the first and second electrodes and accordingly an amount of change in the facing area per unit load become sufficiently large, without the need of making the transverse sectional dimensions of the electrodes large, thereby providing a load sensor which is compact in size and high in measurement accuracy.
In the present invention, preferably, the first and second electrodes are each formed into a hollow cylinder shape which is open at its opposite end faces. The first holding member is comprised of a hollow outer tube having a peripheral wall and an end wall formed integrally therewith at one end of the peripheral wall. The second holding member is comprised of a hollow inner tube disposed in the hollow outer tube and having a peripheral wall and an end wall formed integrally therewith. The outer and inner tubes are arranged for relative motion along the longitudinal axis of the load sensor. The first electrode is attached to the inner peripheral face of the peripheral wall of the outer tube, and the second electrode is attached to the outer peripheral face of the peripheral wall of the inner tube.
With this preferred embodiment, the first and second holding members positively hold the first and second electrodes for relative motion, with the electrode distance maintained constant. Thus, an amount of relative motion between the first and second electrodes caused by the application of load and accordingly an amount of change in the facing area between the electrodes accurately correspond to the magnitude of the applied load, whereby the measurement accuracy, in particular, of the load sensor is improved.
In the preferred embodiment, preferably, the elastic member is comprised of a coil spring having a coil portion and two extension portions extending from the coil portion outward along the longitudinal axis of the load sensor. The coil portion of the coil spring is disposed within the interior space of the second electrode, and the two extension portions of the coil spring are coupled to the end walls of the outer and inner tubes, respectively. The load to be measured by the load sensor is applied to at least one of the two extension portions of the coil spring.
With this preferred embodiment, when load is applied to one or both of the two extension portions of the coil spring, the coil portion of the coil spring is elastically deformed by an amount corresponding to the magnitude of the load. The amount of elastic deformation of the coil portion accurately indicates the magnitude of the applied load, thereby contributing to improved measurement accuracy of the load sensor. Further, the coil spring is high in strength, and the load is applied to one or both of the two extension portions of the coil spring which are respectively coupled to the end walls of the outer and inner tubes. This contributes to the improvement of the long-term reliability of the load sensor.
In the preferred embodiment having the outer and inner tubes, preferably, the outer tube has an end wall extension portion extending radially outwardly from the end wall of the outer tube beyond the peripheral wall of the outer tube, an outer shaft portion extending from the end wall of the outer tube outward along the longitudinal axis of the load sensor, and an inner shaft portion fitted in the interior space of the inner tube and extending from the end wall of the outer tube inward along the longitudinal axis of the load sensor. The inner tube has an end wall extension portion extending radially outwardly from the end wall of the inner tube beyond the peripheral wall of the inner tube, and an outer shaft portion extending from the end wall of the inner tube outward along the longitudinal axis of the load sensor. The elastic member is comprised of a coil spring disposed radially outwardly of the peripheral wall of the outer tube and having opposite ends thereof associated with the end wall extension portions of the outer and inner tubes, respectively. The load to be measured by the load sensor is applied to at least one of the outer shaft portions of the outer and inner tubes.
Alternatively, the outer tube has an outer shaft portion extending from the end wall of the outer tube outward along the longitudinal axis of the load sensor, an inner shaft portion fitted to the interior space of the inner tube and extending from the end wall of the outer tube inward along the longitudinal axis of the load sensor, and a second end wall extending radially inwardly from another end of the peripheral wall of the outer tube and formed with an opening permitting the inner tube to pass therethrough. The inner tube has an outer shaft portion extending from the end wall of the inner tube outward along the longitudinal axis of the load sensor, and a second end wall extending radially outwardly from another end of the peripheral wall of the inner tube and formed with an opening permitting the inner shaft portion of the outer tube to pass therethrough. The elastic member is comprised of a coil spring disposed between the inner peripheral face of the outer tube and the outer peripheral face of the inner tube and having opposite ends thereof associated with the second end walls of the outer and inner tubes, respectively. The load to be measured by the load sensor is applied to at least one of the outer shaft portions of the outer and inner tubes.
In either of the two just-mentioned preferred embodiments, one or both of the outer shaft portions of the outer and inner tubes of the load sensor receive the load, and hence no load is applied to the first and second electrodes respectively attached to the outer and inner tubes. Accordingly, the long-term reliability of the load sensor is improved. Further, an amount of relative motion between the outer and inner tubes upon application of load is regulated by the coil spring that is associated with the outer and inner tubes and elastically deformed by an amount accurately corresponding to the magnitude of the applied load. Thus, a change in the facing area between the first and second electrodes and therefore a change in the capacitance between the electrodes accurately indicate the magnitude of the applied load, thereby contributing to the improvement of the measurement accuracy of the load sensor. Furthermore, since the inner shaft portion of the outer tube is fitted in the interior space of the inner tube, the outer and inner tubes make a smooth relative motion with the electrode distance kept constant, thereby contributing to the improvement in measurement accuracy.
In the present invention, preferably, the first and second electrodes are each formed into a hollow cylindrical shape which is open at its opposite end faces. At least one of the first and second electrodes is configured to have a self-shape-sustaining ability. The first and second holding members are comprised of insulating bushings fitted in respective one end portions of the first and second electrodes.
With this preferred embodiment, the first and second electrodes of a hollow cylinder shape are held by means of the insulating bushings respectively fitted thereto, thereby making it possible to reduce manufacturing costs of the load sensor.
In the just-mentioned preferred embodiment, preferably, the elastic member is comprised of a coil spring having a coil portion and two extension portions extending from opposite ends of the coil portion outward along the longitudinal axis of the load sensor. The coil spring is disposed within the interior space of the second electrode, and the two extension portions of the coil spring are respectively coupled to the insulating bushings. The load to be measured by the load sensor is applied to at least one of the two extension portions of the coil spring.
With this preferred embodiment, load is applied to one or both of the two extension portions of the coil spring which are respectively coupled to the two insulating bushings. This contributes to the improvement in the long-term reliability of the load sensor. Further, an amount of elastic deformation of the coil portion of the coil spring upon application of load accurately represents the magnitude of the applied load, contributing to the improvement in the measurement accuracy of the load sensor.
More preferably, the load sensor further includes an insulating spacer disposed between the inner peripheral face of the first electrode and the outer peripheral face of the second electrode. The insulating spacer is fixed to the inner peripheral face of the first electrode or to the outer peripheral face of the second electrode.
With this preferred embodiment, the first and second electrodes are positively electrically insulated from each other and the electrode distance is held constant by means of the insulating spacer disposed therebetween, thereby contributing to the improvement in measurement accuracy and long-term reliability of the load sensor.
In the present invention, preferably, the elastic modulus of the elastic member is determined such that an amount of elastic deformation per unit load becomes sufficiently large in a load range within which the load to be measured varies.
With this preferred embodiment, an amount of relative motion per unit load between the first and second electrodes and therefore an amount of change in the facing area between the electrodes, i.e., an amount of change in capacitance between the electrodes, become sufficiently large, improving the measurement accuracy of the load sensor.